The present invention relates to the removal of water, carbon dioxide and nitrous oxide from an air stream prior to cryogenic air separation.
The cryogenic separation of air requires a pre-purification step for the removal of both high boiling and hazardous materials. Principal high boiling air components include both water and carbon dioxide. If removal of these impurities from ambient feed air is not achieved, then water and carbon dioxide will freeze out in cold sections of the separation process such as heat exchangers and the LOX sump. This will cause pressure drop, flow variations and operational problems. Various hazardous materials have also to be removed including acetylene and other hydrocarbons. The high boiling hydrocarbons are a problem because they will concentrate in the LOX section of the column, resulting in a potential explosive hazard.
It is known that oxides of nitrogen should be removed also. A minor air component is nitrous oxide N.sub.2 O, which is present in ambient air at about 0.3 ppm. It has similar physical properties to carbon dioxide and therefore presents a potential operation problem because of solids formation in the column and heat exchangers of the cryogenic distillation apparatus. In addition, nitrous oxide is known to enhance combustion of organic materials and is shock sensitive. As such nitrous oxide also presents a safety hazard. Ethylene is a further impurity in air which is desirably removed prior to cryogenic air separation.
The pre-purification of air is usually conducted by adsorptive clean up processes. These may operate by thermal swing adsorption (TSA) as described in U.S. Pat. Nos. 4,541,851 and 5,137,548 or by pressure swing adsorption (PSA) as described in U.S. Pat. No. 5,232,474.
Wenning (`Nitrous oxides in Air Separation Plants` U. Wenning, Proceedings from MUST 96, pp 79-89) describes how carbon dioxide can displace already adsorbed nitrous oxide from a zeolite adsorbent, causing breakthrough of nitrous oxide at a concentration greater than that in ambient air.
No solution to the problem is offered, but Wenning indicates that there may be a need for a search for a more suitable adsorption material for nitrous oxide in the future.
U.S. Pat. No. 4,933,158 suggests that various natural zeolites may be superior to synthetic zeolites for adsorbing nitrous oxide, carbon dioxide and N.sub.2 F.sub.2 from NF.sub.3.
EP-A-0284850 discloses the use of multivalent cation exchanged zeolites for removing water and carbon dioxide from air prior to air separation. It is remarked that other impurities including nitrogen oxides and olefins can be removed also, although no data are presented. In the preferred practice of the invention, the multivalent cation is barium or strontium, and in particular has an ionic radius greater than Ca.sup.2+. However, it is indicated that although not preferred, calcium may be used. The zeolite itself may be 13X. The benefit obtained from the use of the multivalent cation exchanged zeolite is that water can be removed during regeneration at a low temperature. Accordingly, it will be essential that the multivalent cation exchanged zeolite is used for water adsorption.
A further stated advantage of using the multivalent cation exchanged zeolites is that they are said to be able to adsorb more carbon dioxide. Plainly, therefore, the cation exchanged zeolite will need to be used for carbon dioxide adsorption as well as water adsorption. The extent to which a Ca exchanged 13X zeolite would adsorb nitrous oxide in particular is not disclosed.